1. Field of the Invention
The present invention relates to photomasks such as reticles and a method for producing a semiconductor device using photomasks.
2. Description of the Related Art
Photolithography is well known as a technique used in the fabrication of semiconductor devices to transfer a desired pattern onto a semiconductor substrate. The desired pattern is formed initially on the photomask, and an image of the desired pattern is transferred onto a surface of the semiconductor substrate by exposing the semiconductor substrate in combination with the photomask. Typically, fabrication of semiconductor devices requires a set of photomasks, or mask layers each photomask having a different pattern to be utilized to form a different layer on the semiconductor substrate. In the case where a step-and-repeat camera, which is a kind of projection camera, is utilized as an exposure apparatus, the pattern formed on the photomask is transferred to the semiconductor substrate in a step-and-repeat fashion. The pattern on the photomask itself is sometimes over-sized and is optically reduced when formed on the semiconductor substrate. Such a photomask is often referred to as a reticle.
As an example of a photolithographic process, the production of a solid state imaging device is explained in detail. FIG. 1 shows, in part, two patterns which are formed on a semiconductor substrate 101 in two different steps of fabrication. In FIG. 1, a channel stop pattern 103 and a shield metal pattern 104 (shown with cross-hatching) overlapping the channel stop pattern 103 are shown. The channel stop pattern 103 defines a region for isolating a plurality of pixels in the solid state imaging device. The shield metal pattern 104 defines a plurality of windows 104' corresponding to pixels through which light penetrates. The channel stop pattern 103 and the shield metal pattern 104 are formed on a region 105 of the semiconductor substrate 101 defining an area of one chip of the solid state imaging device. The chip has a length 1. The semiconductor substrate 101 may include a plurality of identical chips as shown in part.
In order to form the channel stop pattern 103 or the shield metal pattern 104 on the semiconductor substrate 101, a respective pattern which is five times (5.times.) as large, for example, as the channel stop pattern 103 or the shield metal pattern 104 is formed on a reticle. In the event the solid state imaging device being produced has a substantially long length 1, however, the pattern to form the channel stop pattern 103 or the shield metal pattern 104 may be too large to be formed as a single continuous pattern on the reticle. For example, if the solid state imaging device has the length 1 equal to 20 mm, the 5.times. reticle must be able to accommodate thereon a pattern having the length equal to 100 mm. However, the reticle may not have an available area long enough to accommodate such a large pattern thereon.
In such case, each pattern utilized in forming each corresponding layer is divided into a plurality of separate, non-continuous portions. These portions, referred to herein as sub-patterns, are combined on the substrate to fabricate one chip. As is shown in FIG. 1, the channel stop pattern 103 is divided into sub-patterns 103A, 103B, and 103C at positions 106 and 107. The shield metal pattern 104 is divided into sub-patterns 104A, 104B, and 104C, also at positions 106 and 107. The channel stop pattern 103 and the shield metal pattern 104 are divided at the same positions as is conventional because of ease in designing the patterns, manufacturing the reticles, and checking misalignment of the sub-patterns.
FIG. 2 schematically shows an arrangement of the sub-patterns of the channel stop pattern 103 on the reticle 102. It is noted that although the reticle 102 has a pattern which is five times as large as the channel stop pattern 103, the reticle 102 is referred to herein simply as including the channel stop pattern 103 for clarity. It will be appreciated that other orders of reduction can be utilized without departing from the scope of the invention.
The reticle 102 has composite mask patterns 108 and 109 thereon. The composite mask pattern 108 consists of the pattern in which two sub-patterns 103A and two sub-patterns 103C are each arranged adjacently in a y-axis direction. The composite mask pattern 109 consists of two sub-patterns 103B arranged in the y-axis direction. Overlap patterns 111A, 111B, and 111C are formed adjacent to the sub-patterns 103A, 103B, and 103C, respectively. The overlap patterns 111A, 111B, and 111C provide a degree of overlap between the respective sub-patterns to provide better continuity in the channel stop pattern 103 when the sub-patterns 103A, 103B, and 103C are combined to form the complete channel stop pattern 103. Each of the sub-patterns 103A, 103B, and 103C is isolated by scribe regions 110. A border region 116 separates the composite mask patterns 108 and 109. The sub-patterns 104A, 104B, and 104C are arranged on another reticle 102 in the same way as are sub-patterns 103A, 103B, and 103C in FIG. 2, except that the sub-patterns 104A, 104B, and 104C appear in place of the sub-patterns 103A, 103B, and 103C, respectively.
In order to expose the semiconductor substrate 101 using the appropriate reticle 102 for each respective layer, a step-and-repeat camera, commonly referred to as a stepper, is utilized. The stepper has an offset means for changing projection coordinates, blind means for shielding a part of the reticle, and step-and-repeat means for exposing a substrate in step-and-repeat fashion.
FIG. 3A schematically shows a channel stop pattern 103 formed on the semiconductor substrate 101 using the sub-patterns 103A, 103B, and 103C. Referring to FIGS. 2 and 3A, the method of forming the channel stop pattern 103 on the semiconductor substrate 101 using the corresponding reticle 102 of FIG. 2 and the stepper is now explained. The semiconductor substrate 101 is exposed using the reticle 102 whereby the composite mask pattern 109 is shielded using the blind means of the stepper, and the sub-patterns 103C and 103A are irradiated and thereby transferred on the semiconductors substrate 101.
Then, the projection coordinates are moved using the offset means, and the semiconductor substrate 101 is exposed using the reticle 102 of FIG. 2 in which the composite mask pattern 108 is shielded, so that the sub-pattern 103B is formed between the sub-patterns 103A and 103C previously formed on the semiconductor substrate 101. The overlap patterns 111A and 111B overlap with parts of the sub-patterns 103B and 103A, respectively. A border 112 is formed between an edge of the sub-pattern 103A and an edge of the sub-pattern 103B. The border 112 is disposed at the position 106 shown in FIG. 1. The overlap patterns 111B and 111C also overlap with parts of the sub-patterns 103C and 103B, respectively. A border 113 is formed between the other edge of the sub-pattern 103B and an edge of the sub-pattern 103C. The border 113 is disposed at the position 107 shown in FIG. 1. As a result, the channel stop pattern 103 consisting of the sub-patterns 103A, 103B, and 103C is formed on the semiconductor substrate 101.
As previously mentioned, the shield metal pattern 104 is also divided into the sub-patterns 104A, 104B, and 104C. The sub-patterns 104A, 104B, and 104C are disposed on another reticle (not shown) in the same manner as the sub-patterns 103A, 103B, and 103C for the channel stop pattern 103. As is shown in FIG. 3B, the shield metal pattern 104 consisting of the sub-patterns 104A, 104B, and 104C is formed as a layer on the semiconductor substrate 101 on top of the channel stop pattern with resultant borders 112 and 113 at positions 106 and 107 (FIG. 1), respectively.
Accordingly, the aforementioned method results in each layers on the chip including the borders 112 and 113. FIG. 4 represents an enlarged view near an exemplary border 112 in an actual device. FIG. 5, in comparison, shows an enlarged view near the border 112 according to an ideal device. As is shown in FIG. 4, because transferring the sub-patterns 103A and 103B needs different alignments of the reticle in relation to the semiconductor substrate 101, a relative misalignment between the sub-pattern 103B and the sub-pattern 103A has arisen. In particular, the sub-pattern 103B is misaligned in both the x-axis and y-axis directions relative to the sub-pattern 103A at the border 112. The sub-pattern 104B also is misaligned relative to the sub-pattern 104A at the border 112. Moreover, the pattern width m of the sub-pattern 104A may be different from the pattern width n of the sub-pattern 104B because of lens distortion in the stepper.
Each light receiving portion of the solid state imaging device has an area defined by the channel stop pattern 103 and the shield metal pattern 104. Because of the misalignments mentioned above, however, a light receiving portion 114, for example, has a different area from that of a light receiving portion 115. The difference in the area in the light receiving portions degrades the uniformity characteristics of the device as will be appreciated. Specifically, the light detecting sensitivity, saturation voltage, and the like become disproportionate between the light receiving portions 114 and 115, for example.
In FIG. 4, it is shown that the sub-patterns 103A and 103B of the channel stop pattern 103 and the sub-patterns 104A and 104B of the shield metal pattern 104 misalign at the same border 112. However, it will be appreciated that the sub-patterns of all layers in the solid state imaging device, including those other than the channel stop pattern 103 and the shield metal pattern 104, may misalign at the same border 112. The misalignment of each of these layers is accumulated at the border 112, and device characteristics of the light receiving portion 115 are even more. different from those of the light receiving portion 114 on the other side of the border.
As is explained above, according to a conventional method for producing a semiconductor device, each misalignment occurs at the same position, that makes the non-uniformity of the device characteristics in one chip increasing. The lens distortion in the stepper causes deformation of desired patterns to be formed on the semiconductor substrate, that also degrades the uniformity of the device characteristics. The present invention can solve the aforementioned shortcomings associated with the conventional method of producing a semiconductor device and provide a method of producing the semiconductor device having uniform device characteristics.